Improve Error Messages

## Description
Currently, the error messages are not efficient and developer friendly. It's not easy to understand what's wrong. Below are few instances

- getDeviceFactor - When there is not device factor, the error is `The operation couldn’t be completed. (mpc_core_kit_swift.CoreKitError error 0.)`. This error doesn't make me understand as a developer what's the issue or even show meaningful message to the user.
- enableMFA - When MFA is already enabled, it throws `The operation couldn’t be completed. (mpc_core_kit_swift.CoreKitError error 24.)` instead of throwing MFA is already enabled.

Start by locating the getDeviceFactor and enableMFA implementations, then find the CoreKitError definitions or mapping used for their failures. Compare the current errors with the two cases described and determine the expected developer- and user-facing messages; done means those conditions produce meaningful, distinguishable errors.

Written by the indexing model from the issue text.